This invention relates to a furance assembly for electrically heating a high temperature and high pressure process. This invention is particularly concerned with a multi-stage furnace assembly useful for hot isostatic pressing.
Electrically heated furnaces used in relatively large, high temperature (e.g., up to 3500.degree. F) and high pressure (e.g., up to 50,000 p.s.i.), process vessels, such as hot isostatic presses, typically have been constructed as multi-stage furnaces. This has been done so that the heating of different stages or portions of the interiors of such high temperature and pressure, process vessels, along their longitudinal axes, can be independently varied. Being able to vary the heating of discrete stages of the furnaces has made it possible to compensate for the significant temperature differentials which generally occur in such process vessels, due principally to unequal heat losses from different portions of the process vessels and to non-uniform convection heat transfers between portions of the process vessels. As a result, one has been able to minimize the temperature fluctuations within process vessels, along the longitudinal axes thereof, under a wide variety of process conditions.
Multi-stage furnaces for high temperature and high pressure, process vessels generally have included a plurality of resistance heating elements. The resistance heating elements have been disposed in different stages of the furnaces, along the common longitudinal axes of the furnaces and their surrounding process vessels. The heating elements also have been provided with a separate power feed line to each heating element, separate means for regulating the power provided to each heating element, and separate temperature sensing devices for each stage of the furnaces containing a separate heating element. As a result, the multi-stage furnaces have been adapted to provide power inputs to the individual heating elements, in the different stages of the furnaces, sufficient for the varying temperature and power needs of the overall process operations being carried out and adjusted to correct any untoward temperature variations between stages of the furnaces. Hence, it has been possible with such multi-stage furnaces to carryout relatively large, high temperature and pressure, process operations with uniform temperature conditions in substantially all the stages of the process vessels being utilized.
However, serious problems have been encountered in constructing multi-stage furnaces having a plurality of resistance heating elements for process operations such as hot isostatic pressing. Typically, furnaces for such high temperature and high pressure processes have had to be designed to occupy only very limited spaces. This has been due to the extremely high cost of space in process vessels suited to high temperatures and pressures. Because of the severe space restrictions in high temperature, high pressure, process vessels, it has been very difficult to provide adequate structural support for all of the elements of the multi-stage furnaces. It also has been rather difficult to provide adequate electrical insulation for the separate resistance heating elements of the several stages of multi-stage furnaces and for the separate electrical feed lines to the heating elements in the restricted space available in such process vessels. Furthermore, it has been quite difficult to assemble such furnaces and, when necessary, to repair them because of the confined space in which the furnace elements have been located. Furnace assemblies have been sought therefore which are suitable for the multi-stage heating of high temperature, high pressure, process vessels and which occupy the minimum amount of space in the vessels consistent with proper electrical insulation and structural support for the furnace elements and ease of assembly and repair of the furnace elements.
Also because of the severe space restrictions in vessels for high temperature, high pressure processes, it has been extremely difficult to protect the elastomeric electrical insulation utilized for insulating electrical feed lines as they enter the process vessels from deterioration due to the high temperatures generated by the furnace assemblies in such vessels. Means have been sought therefore for keeping the streams of hot gases from the furance assemblies away from the elastomeric electrical insulation in the process vessels. Means also have been sought for minimizing the detrimental effects upon the elastomeric insulation of heat energy radiated from the heating elements of the furance assemblies.